Examples of high-frequency circuit boards include a circuit board that uses a microstrip-type high-frequency line. The microstrip-type circuit board has a ground conductor formed on a back surface of a high-frequency signal line disposed on a front surface. These high-frequency circuit boards include an end connection portion (pad) for conducting a signal with an electrode of an electronic component or another circuit board.
FIG. 4 is a plan view illustrating an example of a circuit board 100 including a conventional microstrip line type differential signal line (refer to Patent Document 1, for example).
As illustrated in FIG. 4, a differential signal line 103 made of a pair of microstrip lines including a first microstrip line 101 and a second microstrip line 102 is formed on a front surface of the board 100. A ground conductor 104 (not illustrated) is formed on a back surface of the circuit board 100, in a corresponding portion in which the differential signal line 103 is formed.
Pads 101a, 102a which are wider than the first and second microstrip lines 101, 102 are formed on a terminating portion (lower portion in FIG. 4) of the circuit board 100. The pads 101a, 102a are provided to facilitate a connection. Then, the pads 101a, 102a are connected to an electrode or the like of an electronic component, thereby causing the circuit board 100 to transmit a high-frequency electric signal to or from the electronic component.
First and second relay lines 101b, 102b are provided to the differential signal line of Patent Document 1, extending between the first and second microstrip lines 101, 102 and the pads 101a, 102a. The first and second relay lines 101b, 102b have a line width that is less than the line width of the first and second microstrip lines 101, 102. In the pad 101a, 102a portions which have a wider line width, a high-frequency impedance decreases. The first and second relay lines 101b, 102b are provided to compensate for this decrease and improve high-frequency impedance matching.